Adaptation to hypoxia (low oxygen tension) is essential in developmental, physiological and pathological processes including neural and cardiac development, hematopoiesis, angiogenesis and tumorigenesis. In particular, hypoxia is associated with tumor angiogenesis, progression and metastasis. Hypoxic induction of angiogenesis is necessary for tumor formation and progression, and results from the induction of the vascular endothelial growth factor (VEGF). Hypoxic regions within tumors contain cells with diminished apoptotic potential and become resistant to radiotherapy. Recent studies have demonstrated that hypoxia-inducible factor 1 (HIF1), a heterodimeric basic helix-loop-helix-PAS transcription factor, plays a key role in response to hypoxia during angiogenesis and tumorigenesis. HIF1 is composed of two subunits, HIF1a and ARNT, and is required for transcriptional up-regulation of a growing number of hypoxia responsive genes, e.g., erythropoietin, VEGF, and inducible nitric oxide synthase. Furthermore, HIF1a has been shown to be necessary for hypoxic stabilization of wild-type tumor suppressor p53, consistent with the notion that HIF1a is involved in hypoxia-mediated apoptosis and cell proliferation. Despite the significant advancement in recent years, little is known about the molecular basis of HIF1 activation. Our objective is to understand the molecular mechanisms underlying HIF1 activation. Hypoxia-induced HIF1 activity involves a multi-step temporal process including post-translational stabilization, nuclear translocation and transcriptional activation of HIF1a. HIF1a stabilization results from inhibition of the ubiquitin-proteasomal degradation pathway that is associated with tumor suppressor genes, e.g., the von Hippel-Lindau protein (pVHL) and p53, functioning as E3 ubiquitin ligases. Stabilized HIF1a exerts its transcriptional activity by recruiting the transcriptional co-activator and integrator p300/CBP. Our research focuses on the identification of critical residues involving HIF1a degradation and transcriptional activation. Recently, we have developed a RAndom Mutagenesis Screen in Yeast (RAMSY) approach for efficient identification of critical residues in protein-protein interactions. This method has enabled us to uncover the molecular mechanism by which HIF1a interacts with p300/CBP, and therefore should provide an applicable means for uncovering mechanisms of broad biological processes through the rapid identification of molecular determinants.